There are many applications in which it is desirable to enhance transition speed of a signal that is transmitted on a transmission line to a transducer or receiver. It is also important to provide this enhancement while expending the least amount of power, or no extra power at all. In the following, the particular example of a current delivered the write coil of a magnetic data storage device will be used as an illustrative example, which also includes the additional challenge of maintaining the write current level in the coil while lowering power dissipation in the system. The current invention however has broader application as will be appreciated by those of skill in the art.
Magnetic data storage devices include front-end circuits 10 (see FIG. 1) that read and write the data to the storage media, such as used in hard disk drive (HDD) storage devices, typically include one or more pairs of magnetic transducers 23, 24 for reading and writing magnetic transitions in magnetic media. The slider 13 includes the read and write transducer elements. The read transducer element 23 and write transducer element 24 are also called the read head and write head respectively. The slider 13 is mounted on an actuator arm (not shown) which mechanically positions the transducers over selected tracks on the magnetic media on rotating disks (not shown). The electrical signals to and from the read and write transducer elements are processed by appropriate electronic circuitry in read amplifier 21 and write driver 22 which are connected through electrically conductive paths 25A, 25B, 26A, 26B. The read/write channel 20 reads data from the read amplifier 21 and supplies data signals to write driver 22.
The write transducer 24 writes digital information on the rotating disk media on the disk by creating magnetic flux reversals that corresponds to write signal. The terms write head, write transducer, write element and write coil will be used interchangeably herein. The electrical current for the write coil is supplied by write driver 22. The direction of the current flow in the coil, which determines the polarity of generated magnetic field, is typically controlled by four transistor switches (not shown) in an “H-bridge” arrangement in the write driver circuit.
The spectral content of the write signals tend to have higher frequency components than the read signals due to the square wave nature of the write voltage (or current) signals generated in the write driver. To achieve high-data rates, low loss high bandwidth transmission lines are used between the write driver and the write elements. In addition, the write transducer's magnetic switching speed can be relatively slow compared to the desired data rate, which requires a boost or overshoot during write signal reversals. In the interest of avoiding undesirable signal reflections back to the write element, prior art write drivers deliver current to the coil through transmission lines with static (unswitched) source termination. Write drivers can generally be of voltage-type or current-type. Voltage-type write drivers with standard statically terminated lines are required to operate with voltage supply levels of the order of 2ZoIw, where Zo is the characteristic impedance of transmission line connecting the write driver to the coil, and Iw is the current required by the coil. Current-type write drivers use shunt-type line termination and therefore need, at launch, to be able to provide current levels of the order of 2Iw to achieve Iw in the coil.
Designing for low power typically entails lowering the voltage of operation. Since the amount of current for proper magnetic recording to be delivered to the write coil in a magnetic recording systems is fixed by physical constraints in head and media design, lowering the voltage of operation involves lowering the characteristic impedance of the trace interconnections between the write driver and the write coil. This requires specific design to enable low insertion loss, large bandwidth and adequate physical widths of the low impedance transmission lines.
Prior art design techniques match the write driver's output impedance value to be equal or a small percentage greater than the characteristic impedance of the transmission line. If the write driver's impedance is significantly mismatched with the characteristic impedance of the transmission line, undesirable signal reflections can occur in prior art designs. The reflected signal can interfere with the transmitted signal, causing distortion and degrading signal integrity. In the prior art the undesirable reflected signal is, therefore, terminated at the write driver's output, which is also called source-side termination.
FIG. 2 is a conceptual illustration of a prior art current-source-type write driver 22a with current sources 34, 35 and a receiver 24A modeled as simple LRC elements. The write driver 22a could be used in a disk drive, for example, and receiver 24A can be a component similar to an inductive write coil in a disk drive. The connections or leads 26C, 26D to the receiver 24A have characteristic transmission line impedances of Z1 and Z2 which are each one-half of the total Zo. Transistor switches 31a-d (in the “H-bridge” arrangement) control the direction (polarity) of current flow through the receiver 24A. The transistor switches are shown in symbolic form to indicate the open (high impedance) or closed (conducting/low impedance) state. In an H-bridge circuit, one leg or the other of the bridge is supplying current into the receiver 24A. In the case of a disk drive, the transition from one polarity to the other records information in the magnetic media. As shown, switches 31b and 31c are open while 31a and 31d are closed to apply current in one direction through the receiver 24A. The state of the switches is reversed to apply current in the opposite direction through the receiver 24A. In this configuration resistors 29a, 29b provide termination. In a fully differential operation, the node “C” in FIG. 2 behaves as a virtual ground.
U.S. Pat. No. 4,414,480 to Zasio describes an output circuit designed to take advantage signal reflection for a nonterminated line where the receiving circuit appears as an open circuit to the transmission line. Since the output signal is completely reflected when it reaches the output end of the transmission line, the amplitude of the signal at the receiving circuit can be double that of the initial signal provided by the output circuit. This is due to the reflected signal combining with the incident signal. This cuts the drive current requirements of the output circuit in half. However, if the signal were to be reflected by the output circuit, this could interfere with the detection of switching transitions. In order to avoid this, the output circuit is designed so that its output impedance is approximately equal to the characteristic impedance of the transmission line. Therefore, the output circuit provides a series termination for the input end of the transmission line and will completely absorb the reflected signal.
U.S. Pat. No. 6,671,113 to Klaassen, et al. describes a write driver circuit that reduces the reversal time for the current through the inductive recording head, The write driver output stage includes a source-side termination circuit having output impedance ZS, wherein the source-side termination circuit output impedance ZS is substantially equal to ZO and the source strength SO (which represents current drive capability) of the write driver at the input of the interconnect circuit is temporarily enlarged after every polarity reversal of the write signal for a predetermined time duration. Klaassen's source-terminated current-type write driver embodiment shows the write driver source strength enhancement is obtained by connecting a short current pulse to the input terminals of the integrated lead suspension (ILS) so that a higher voltage step is created across the ILS input terminals during a current reversal.
U.S. Pat. No. 6,721,115 to John Price, Jr. describes a technique said to provide a current boost during the switching transition in a current-type write driver by boosting pull-up current during a write current transition.
Note that both U.S. patents above illustrate the state of art, in which transition speed is enhanced at the expense of augmenting power dissipation in the write driver at the transition events.
Data rates in commercial hard disks are expected to move above 3 Gbps in the next few years, and the continued increase in data rates pose a power and heat challenge for designers. Write drivers are typically required to provide about 100 mA of current and use 5V power supplies, or approximately 0.5 W of instantaneous power. Since mobile HDD applications require total power dissipation below 2.5 W including motors, etc., there is a need to provide writing capabilities in hard drives at the progressive higher rates at the lowest power levels possible.